Full Idea
We judge to be false that which involves a contradiction, and true that which is opposed or contradictory to the false.
Gist of Idea
Falsehood involves a contradiction, and truth is contradictory of falsehood
Source
Gottfried Leibniz (Monadology [1716], §31)
Book Reference
Leibniz,Gottfried: 'Philosophical Writings', ed/tr. Parkinson,G.H.R. [Dent 1973], p.184